This invention relates to a power transmission belt, particularly one having drive teeth for use in positive drive systems.
The use of fiber reinforcement in the tooth portion of a power transmission belt is well known. The long fibers act to mechanically disperse localized stressing in the tooth portion of the belt thus minimizing heat build up and localized shearing stresses. In all known prior belt constructions it was found desirable for there to be a non-fiber reinforced backing layer in the tension section of the belt lying on the opposite side of the inextensible tensile bearing member of the belt.
The ratio of average length to diameter ratio shall be referred to as L/D ratio.
Prior art belt constructions generally utilize fiber having a length sufficient to assume an orientation during mechanical mixing and preparation of the rubber compound. These prior art belts utilize L/D ratios greater than 10 and preferably into the 30 to 40 range. During calendering or extrusion of the fully compounded fiber reinforced rubber compound, the relatively long length fibers tend to orient themselves in the machine direction or perpendicular to the calender rolls or extruder die opening. This orientation in a direction generally parallel to the tensile bearing member of the belt was generally considered to be advantageous since most shear stressing on the belt was in that direction.
The previously mentioned non-fiber reinforced backing compound used above the tensile bearing member has been found to exhibit a tendency to delaminate during high temperature service at the interface between the non-fiber-loaded backing compound and the fiber loaded tooth stock in the prior art belts. This tendency to delaminate at the interface between dissimilar rubber compounds leads to premature failure of the belt and is generally attributed to problems in knitting or adhesion between the two dissimilar stocks. The knitting difficulties are generally attributed to different rheological flow properties and chemical characteristics at the interface which prevail even through the final preparation and curing steps of the belt manufacturing process.
It has been surprisingly and unexpectedly found that the use of a very finely chopped fibrous material commonly called flock having an L/D ratio less than 10 yields a rubber composition which is useable throughout the entirety of the elastomeric body of a positive drive belt. This flock material eliminates the necessity of providing a separate non-fiber reinforced backing layer. The entire elastomeric composition of the belt is a single rubber compound having homogeneously dispersed therein the low L/D flock material. This flock is a carefully selected type of fiber having extremely short lengths and very low ratios of length to diameter. Preferred L/D ratios are below 10/1 and more preferred are flocks below 5/1. The ability to use a single type of fiber throughout the whole elastomeric body of the belt eliminated the many fold problems associated with having dissimilar rubber compounds in the belt. The homogenous composition of this belt eliminated the tendency to delaminate during service along the interface line between the fiber reinforced stocks and the non-fiber reinforced backing compounds of the known prior art belts. This ability to resist delamination yields longer service life. Further, the uniform distribution of flock throughout the tooth portion of the positive drive belt gives substantially improved shear resistance to the tooth compared to non-fiber reinforced tooth stocks. This also yields longer service lives and lower heat build up during service.